protobuf3_12: 3.12.3 -> 3.12.4

pkgs/development/libraries/protobuf/3.12.nix

···
 { callPackage, ... }:
 
 callPackage ./generic-v3.nix {
-  version = "3.12.3";
-  sha256 = "0q4sn9d6x8w0zgzydfx9f7b2zdk0kiplk8h9jxyxhw6m9qn276ax";
+  version = "3.12.4";
+  sha256 = "1gzvnd0g5hmx5ln39w7p4z4qphw87ksgsa1fgbpvi8d0asmwab2p";
 }